NCERT Textbook: Microbes in Human Welfare | Biology Class 12 - NEET PDF Download

 Page 1


Besides macroscopic plants and animals, microbes are
the major components of biological systems on this earth.
You have studied about the diversity of living organisms
in Class XI. Do you remember which Kingdoms among
the living organisms contain micro-organisms? Which are
the ones that are only microscopic? Microbes are present
everywhere – in soil, water, air, inside our bodies and that
of other animals and plants. They are present even at sites
where no other life-form could possibly exist–sites such
as deep inside the geysers (thermal vents) where the
temperature may be as high as 100
0
C, deep in the soil,
under the layers of snow several metres thick, and in highly
acidic environments. Microbes are diverse–protozoa,
bacteria, fungi and microscopic animal and plant viruses,
viroids and also prions that are proteinacious infectious
agents. Some of the microbes are shown in Figures 8.1
and 8.2.
Microbes like bacteria and many fungi can be grown
on nutritive media to form colonies (Figure 8.3), that can
be seen with the naked eyes.  Such cultures are useful in
studies on micro-organisms.
CHAPTER 8
MICROBES IN HUMAN WELFARE
8.1 Microbes in Household
Products
8.2 Microbes in Industrial
Products
8.3 Microbes in Sewage
Treatment
8.4 Microbes in Production of
Biogas
8.5 Microbes as Biocontrol
Agents
8.6 Microbes as Biofertilisers
2024-25
Page 2


Besides macroscopic plants and animals, microbes are
the major components of biological systems on this earth.
You have studied about the diversity of living organisms
in Class XI. Do you remember which Kingdoms among
the living organisms contain micro-organisms? Which are
the ones that are only microscopic? Microbes are present
everywhere – in soil, water, air, inside our bodies and that
of other animals and plants. They are present even at sites
where no other life-form could possibly exist–sites such
as deep inside the geysers (thermal vents) where the
temperature may be as high as 100
0
C, deep in the soil,
under the layers of snow several metres thick, and in highly
acidic environments. Microbes are diverse–protozoa,
bacteria, fungi and microscopic animal and plant viruses,
viroids and also prions that are proteinacious infectious
agents. Some of the microbes are shown in Figures 8.1
and 8.2.
Microbes like bacteria and many fungi can be grown
on nutritive media to form colonies (Figure 8.3), that can
be seen with the naked eyes.  Such cultures are useful in
studies on micro-organisms.
CHAPTER 8
MICROBES IN HUMAN WELFARE
8.1 Microbes in Household
Products
8.2 Microbes in Industrial
Products
8.3 Microbes in Sewage
Treatment
8.4 Microbes in Production of
Biogas
8.5 Microbes as Biocontrol
Agents
8.6 Microbes as Biofertilisers
2024-25
150
BIOLOGY
(a)
(b)
(c)
(a) (b)
Figure 8.3 (a) Colonies of bacteria growing in a petri dish;
(b) Fungal colony growing in a petri dish
(a) (b)
(c)
Figure 8.1 Bacteria: (a) Rod-shaped,
magnified 1500X; (b) Spherical
shaped, magnified1500X; (c) A rod-
shaped bacterium showing flagella,
magnified 50,000X
Figure 8.2 Viruses: (a) A bacteriophage; (b)
Adenovirus which causes respiratory
infections; (c) Rod-shaped Tobacco
Mosaic Virus (TMV). Magnified about
1,00,000–1,50,000X
2024-25
Page 3


Besides macroscopic plants and animals, microbes are
the major components of biological systems on this earth.
You have studied about the diversity of living organisms
in Class XI. Do you remember which Kingdoms among
the living organisms contain micro-organisms? Which are
the ones that are only microscopic? Microbes are present
everywhere – in soil, water, air, inside our bodies and that
of other animals and plants. They are present even at sites
where no other life-form could possibly exist–sites such
as deep inside the geysers (thermal vents) where the
temperature may be as high as 100
0
C, deep in the soil,
under the layers of snow several metres thick, and in highly
acidic environments. Microbes are diverse–protozoa,
bacteria, fungi and microscopic animal and plant viruses,
viroids and also prions that are proteinacious infectious
agents. Some of the microbes are shown in Figures 8.1
and 8.2.
Microbes like bacteria and many fungi can be grown
on nutritive media to form colonies (Figure 8.3), that can
be seen with the naked eyes.  Such cultures are useful in
studies on micro-organisms.
CHAPTER 8
MICROBES IN HUMAN WELFARE
8.1 Microbes in Household
Products
8.2 Microbes in Industrial
Products
8.3 Microbes in Sewage
Treatment
8.4 Microbes in Production of
Biogas
8.5 Microbes as Biocontrol
Agents
8.6 Microbes as Biofertilisers
2024-25
150
BIOLOGY
(a)
(b)
(c)
(a) (b)
Figure 8.3 (a) Colonies of bacteria growing in a petri dish;
(b) Fungal colony growing in a petri dish
(a) (b)
(c)
Figure 8.1 Bacteria: (a) Rod-shaped,
magnified 1500X; (b) Spherical
shaped, magnified1500X; (c) A rod-
shaped bacterium showing flagella,
magnified 50,000X
Figure 8.2 Viruses: (a) A bacteriophage; (b)
Adenovirus which causes respiratory
infections; (c) Rod-shaped Tobacco
Mosaic Virus (TMV). Magnified about
1,00,000–1,50,000X
2024-25
151
MICROBES IN HUMAN WELFARE
 In chapter 7, you have read that microbes cause a large number of
diseases in human beings. They also cause diseases in animals and plants.
But this should not make you think that all microbes are harmful; several
microbes are useful to man in diverse ways. Some of the most important
contributions of microbes to human welfare are discussed in this chapter.
8.1 MICROBES IN HOUSEHOLD PRODUCTS
You would be surprised to know that we use microbes or products
derived from them everyday. A common example is the production of
curd from milk. Micro-organisms such as Lactobacillus and others
commonly called lactic acid bacteria (LAB) grow in milk and convert it
to curd. During growth, the LAB produce acids that coagulate and
partially digest the milk proteins. A small amount of curd added to the
fresh milk as inoculum or starter contain millions of LAB, which at
suitable temperatures multiply, thus converting milk to curd, which
also improves its nutritional quality by increasing vitamin B
12
. In our
stomach too, the LAB play very beneficial role in checking disease-
causing microbes.
The dough, which is used for making foods such as dosa and idli is
also fermented by bacteria. The puffed-up appearance of dough is due to
the production of  CO
2
 gas.  Can you tell which metabolic pathway is
taking place resulting in the formation of CO
2
? Where do you think the
bacteria for these fermentations come from?  Similarly the dough, which
is used for making bread, is fermented using baker’s yeast
(Saccharomyces cerevisiae). A number of traditional drinks and foods
are also made by fermentation by the microbes. ‘Toddy’, a traditional
drink of some parts of southern India is made by fermenting sap from
palms. Microbes are also used to ferment fish, soyabean and bamboo-
shoots to make foods. Cheese, is one of the oldest food items in which
microbes were used. Different varieties of cheese are known by their
characteristic texture, flavour and taste, the specificity coming from the
microbes used. For example, the large holes in ‘Swiss cheese’ are due to
production of a large amount of CO
2
 by a bacterium named
Propionibacterium sharmanii.  The ‘Roquefort cheese’ are ripened by
growing a specific fungi on them, which gives them a particular flavour.
8.2 MICROBES IN INDUSTRIAL PRODUCTS
Even in industry, microbes are used to synthesise a number of products
valuable to human beings. Beverages and antibiotics are some examples.
Production on an industrial scale, requires growing microbes in very large
vessels called fermentors (Figure 8.4).
2024-25
Page 4


Besides macroscopic plants and animals, microbes are
the major components of biological systems on this earth.
You have studied about the diversity of living organisms
in Class XI. Do you remember which Kingdoms among
the living organisms contain micro-organisms? Which are
the ones that are only microscopic? Microbes are present
everywhere – in soil, water, air, inside our bodies and that
of other animals and plants. They are present even at sites
where no other life-form could possibly exist–sites such
as deep inside the geysers (thermal vents) where the
temperature may be as high as 100
0
C, deep in the soil,
under the layers of snow several metres thick, and in highly
acidic environments. Microbes are diverse–protozoa,
bacteria, fungi and microscopic animal and plant viruses,
viroids and also prions that are proteinacious infectious
agents. Some of the microbes are shown in Figures 8.1
and 8.2.
Microbes like bacteria and many fungi can be grown
on nutritive media to form colonies (Figure 8.3), that can
be seen with the naked eyes.  Such cultures are useful in
studies on micro-organisms.
CHAPTER 8
MICROBES IN HUMAN WELFARE
8.1 Microbes in Household
Products
8.2 Microbes in Industrial
Products
8.3 Microbes in Sewage
Treatment
8.4 Microbes in Production of
Biogas
8.5 Microbes as Biocontrol
Agents
8.6 Microbes as Biofertilisers
2024-25
150
BIOLOGY
(a)
(b)
(c)
(a) (b)
Figure 8.3 (a) Colonies of bacteria growing in a petri dish;
(b) Fungal colony growing in a petri dish
(a) (b)
(c)
Figure 8.1 Bacteria: (a) Rod-shaped,
magnified 1500X; (b) Spherical
shaped, magnified1500X; (c) A rod-
shaped bacterium showing flagella,
magnified 50,000X
Figure 8.2 Viruses: (a) A bacteriophage; (b)
Adenovirus which causes respiratory
infections; (c) Rod-shaped Tobacco
Mosaic Virus (TMV). Magnified about
1,00,000–1,50,000X
2024-25
151
MICROBES IN HUMAN WELFARE
 In chapter 7, you have read that microbes cause a large number of
diseases in human beings. They also cause diseases in animals and plants.
But this should not make you think that all microbes are harmful; several
microbes are useful to man in diverse ways. Some of the most important
contributions of microbes to human welfare are discussed in this chapter.
8.1 MICROBES IN HOUSEHOLD PRODUCTS
You would be surprised to know that we use microbes or products
derived from them everyday. A common example is the production of
curd from milk. Micro-organisms such as Lactobacillus and others
commonly called lactic acid bacteria (LAB) grow in milk and convert it
to curd. During growth, the LAB produce acids that coagulate and
partially digest the milk proteins. A small amount of curd added to the
fresh milk as inoculum or starter contain millions of LAB, which at
suitable temperatures multiply, thus converting milk to curd, which
also improves its nutritional quality by increasing vitamin B
12
. In our
stomach too, the LAB play very beneficial role in checking disease-
causing microbes.
The dough, which is used for making foods such as dosa and idli is
also fermented by bacteria. The puffed-up appearance of dough is due to
the production of  CO
2
 gas.  Can you tell which metabolic pathway is
taking place resulting in the formation of CO
2
? Where do you think the
bacteria for these fermentations come from?  Similarly the dough, which
is used for making bread, is fermented using baker’s yeast
(Saccharomyces cerevisiae). A number of traditional drinks and foods
are also made by fermentation by the microbes. ‘Toddy’, a traditional
drink of some parts of southern India is made by fermenting sap from
palms. Microbes are also used to ferment fish, soyabean and bamboo-
shoots to make foods. Cheese, is one of the oldest food items in which
microbes were used. Different varieties of cheese are known by their
characteristic texture, flavour and taste, the specificity coming from the
microbes used. For example, the large holes in ‘Swiss cheese’ are due to
production of a large amount of CO
2
 by a bacterium named
Propionibacterium sharmanii.  The ‘Roquefort cheese’ are ripened by
growing a specific fungi on them, which gives them a particular flavour.
8.2 MICROBES IN INDUSTRIAL PRODUCTS
Even in industry, microbes are used to synthesise a number of products
valuable to human beings. Beverages and antibiotics are some examples.
Production on an industrial scale, requires growing microbes in very large
vessels called fermentors (Figure 8.4).
2024-25
152
BIOLOGY
8.2.1 Fermented Beverages
Microbes especially yeasts have been used from
time immemorial for the production of beverages
like wine, beer, whisky, brandy or rum. For this
purpose the same yeast Saccharomyces
cerevisiae used for bread-making and
commonly called brewer’s yeast, is used for
fermenting malted cereals and fruit juices, to
produce ethanol. Do you recollect the metabolic
reactions, which result in the production of
ethanol by yeast? Depending on the type of the
raw material used for fermentation and the type
of processing (with or without distillation)
different types of alcoholic drinks are obtained.
Wine and beer are produced without distillation
whereas whisky, brandy and rum are produced
by distillation of the fermented broth. The
photograph of a fermentation plant is shown in
Figure 8.5.
8.2.2 Antibiotics
Antibiotics produced by microbes are regarded
as one of the most significant discoveries of the
twentieth century and have greatly contributed
towards the welfare of the human society. Anti is
a Greek word that means ‘against’, and bio means
‘life’, together they mean ‘against life’ (in the
context of disease causing organisms); whereas with reference to human
beings, they are ‘pro life’ and not against. Antibiotics are chemical
substances, which are produced by some microbes and can kill or retard
the growth of other (disease-causing) microbes.
You are familiar with the commonly used antibiotic Penicillin. Do you
know that Penicillin was the first antibiotic to be discovered, and it was a
chance discovery?  Alexander Fleming while working on Staphylococci
bacteria, once observed a mould growing in one of his unwashed culture
plates around which Staphylococci could not grow. He found out that it
was due to a chemical produced by the mould and he named it Penicillin
after the mould Penicillium notatum. However, its full potential as an
effective antibiotic was established much later by Ernest Chain and
Howard Florey. This antibiotic  was extensively used to treat American
soldiers wounded in World War II. Fleming, Chain and Florey were awarded
the Nobel Prize in 1945, for this discovery.
Figure 8.5 Fermentation Plant
Figure 8.4 Fermentors
2024-25
Page 5


Besides macroscopic plants and animals, microbes are
the major components of biological systems on this earth.
You have studied about the diversity of living organisms
in Class XI. Do you remember which Kingdoms among
the living organisms contain micro-organisms? Which are
the ones that are only microscopic? Microbes are present
everywhere – in soil, water, air, inside our bodies and that
of other animals and plants. They are present even at sites
where no other life-form could possibly exist–sites such
as deep inside the geysers (thermal vents) where the
temperature may be as high as 100
0
C, deep in the soil,
under the layers of snow several metres thick, and in highly
acidic environments. Microbes are diverse–protozoa,
bacteria, fungi and microscopic animal and plant viruses,
viroids and also prions that are proteinacious infectious
agents. Some of the microbes are shown in Figures 8.1
and 8.2.
Microbes like bacteria and many fungi can be grown
on nutritive media to form colonies (Figure 8.3), that can
be seen with the naked eyes.  Such cultures are useful in
studies on micro-organisms.
CHAPTER 8
MICROBES IN HUMAN WELFARE
8.1 Microbes in Household
Products
8.2 Microbes in Industrial
Products
8.3 Microbes in Sewage
Treatment
8.4 Microbes in Production of
Biogas
8.5 Microbes as Biocontrol
Agents
8.6 Microbes as Biofertilisers
2024-25
150
BIOLOGY
(a)
(b)
(c)
(a) (b)
Figure 8.3 (a) Colonies of bacteria growing in a petri dish;
(b) Fungal colony growing in a petri dish
(a) (b)
(c)
Figure 8.1 Bacteria: (a) Rod-shaped,
magnified 1500X; (b) Spherical
shaped, magnified1500X; (c) A rod-
shaped bacterium showing flagella,
magnified 50,000X
Figure 8.2 Viruses: (a) A bacteriophage; (b)
Adenovirus which causes respiratory
infections; (c) Rod-shaped Tobacco
Mosaic Virus (TMV). Magnified about
1,00,000–1,50,000X
2024-25
151
MICROBES IN HUMAN WELFARE
 In chapter 7, you have read that microbes cause a large number of
diseases in human beings. They also cause diseases in animals and plants.
But this should not make you think that all microbes are harmful; several
microbes are useful to man in diverse ways. Some of the most important
contributions of microbes to human welfare are discussed in this chapter.
8.1 MICROBES IN HOUSEHOLD PRODUCTS
You would be surprised to know that we use microbes or products
derived from them everyday. A common example is the production of
curd from milk. Micro-organisms such as Lactobacillus and others
commonly called lactic acid bacteria (LAB) grow in milk and convert it
to curd. During growth, the LAB produce acids that coagulate and
partially digest the milk proteins. A small amount of curd added to the
fresh milk as inoculum or starter contain millions of LAB, which at
suitable temperatures multiply, thus converting milk to curd, which
also improves its nutritional quality by increasing vitamin B
12
. In our
stomach too, the LAB play very beneficial role in checking disease-
causing microbes.
The dough, which is used for making foods such as dosa and idli is
also fermented by bacteria. The puffed-up appearance of dough is due to
the production of  CO
2
 gas.  Can you tell which metabolic pathway is
taking place resulting in the formation of CO
2
? Where do you think the
bacteria for these fermentations come from?  Similarly the dough, which
is used for making bread, is fermented using baker’s yeast
(Saccharomyces cerevisiae). A number of traditional drinks and foods
are also made by fermentation by the microbes. ‘Toddy’, a traditional
drink of some parts of southern India is made by fermenting sap from
palms. Microbes are also used to ferment fish, soyabean and bamboo-
shoots to make foods. Cheese, is one of the oldest food items in which
microbes were used. Different varieties of cheese are known by their
characteristic texture, flavour and taste, the specificity coming from the
microbes used. For example, the large holes in ‘Swiss cheese’ are due to
production of a large amount of CO
2
 by a bacterium named
Propionibacterium sharmanii.  The ‘Roquefort cheese’ are ripened by
growing a specific fungi on them, which gives them a particular flavour.
8.2 MICROBES IN INDUSTRIAL PRODUCTS
Even in industry, microbes are used to synthesise a number of products
valuable to human beings. Beverages and antibiotics are some examples.
Production on an industrial scale, requires growing microbes in very large
vessels called fermentors (Figure 8.4).
2024-25
152
BIOLOGY
8.2.1 Fermented Beverages
Microbes especially yeasts have been used from
time immemorial for the production of beverages
like wine, beer, whisky, brandy or rum. For this
purpose the same yeast Saccharomyces
cerevisiae used for bread-making and
commonly called brewer’s yeast, is used for
fermenting malted cereals and fruit juices, to
produce ethanol. Do you recollect the metabolic
reactions, which result in the production of
ethanol by yeast? Depending on the type of the
raw material used for fermentation and the type
of processing (with or without distillation)
different types of alcoholic drinks are obtained.
Wine and beer are produced without distillation
whereas whisky, brandy and rum are produced
by distillation of the fermented broth. The
photograph of a fermentation plant is shown in
Figure 8.5.
8.2.2 Antibiotics
Antibiotics produced by microbes are regarded
as one of the most significant discoveries of the
twentieth century and have greatly contributed
towards the welfare of the human society. Anti is
a Greek word that means ‘against’, and bio means
‘life’, together they mean ‘against life’ (in the
context of disease causing organisms); whereas with reference to human
beings, they are ‘pro life’ and not against. Antibiotics are chemical
substances, which are produced by some microbes and can kill or retard
the growth of other (disease-causing) microbes.
You are familiar with the commonly used antibiotic Penicillin. Do you
know that Penicillin was the first antibiotic to be discovered, and it was a
chance discovery?  Alexander Fleming while working on Staphylococci
bacteria, once observed a mould growing in one of his unwashed culture
plates around which Staphylococci could not grow. He found out that it
was due to a chemical produced by the mould and he named it Penicillin
after the mould Penicillium notatum. However, its full potential as an
effective antibiotic was established much later by Ernest Chain and
Howard Florey. This antibiotic  was extensively used to treat American
soldiers wounded in World War II. Fleming, Chain and Florey were awarded
the Nobel Prize in 1945, for this discovery.
Figure 8.5 Fermentation Plant
Figure 8.4 Fermentors
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153
MICROBES IN HUMAN WELFARE
After Penicillin, other antibiotics were also purified from other
microbes. Can you name some other antibiotics and find out their
sources? Antibiotics have greatly improved our capacity to treat deadly
diseases such as plague, whooping cough (kali khansi), diphtheria (gal
ghotu) and leprosy (kusht rog), which used to kill millions all over the
globe. Today, we cannot imagine a world without antibiotics.
8.2.3 Chemicals, Enzymes and other Bioactive Molecules
Microbes are also used for commercial and industrial production of
certain chemicals like organic acids, alcohols and enzymes. Examples of
acid producers are Aspergillus niger (a fungus) of citric acid, Acetobacter
aceti (a bacterium) of acetic acid; Clostridium butylicum (a bacterium) of
butyric acid and Lactobacillus (a bacterium) of lactic acid.
Yeast (Saccharomyces cerevisiae) is used for commercial production
of ethanol. Microbes are also used for production of enzymes. Lipases are
used in detergent formulations and are helpful in removing oily stains
from the laundry. You must have noticed that bottled fruit juices bought
from the market are clearer as compared to those made at home. This is
because the bottled juices are clarified by the use of pectinases and
proteases. Streptokinase produced by the bacterium Streptococcus and
modified by genetic engineering is used as a ‘clot buster’ for removing
clots from the blood vessels of patients who have undergone myocardial
infarction leading to heart attack.
Another bioactive molecule, cyclosporin A, that is used as an
immunosuppressive agent in organ-transplant patients, is produced by
the fungus Trichoderma polysporum. Statins produced by the yeast
Monascus purpureus have been commercialised as blood-cholesterol
lowering agents. It acts by competitively inhibiting the enzyme responsible
for synthesis of cholesterol.
8.3 MICROBES IN SEWAGE TREATMENT
We know that large quantities of waste water are generated everyday in
cities and towns. A major component of this waste water is human excreta.
This municipal waste-water is also called sewage. It contains large
amounts of organic matter and microbes. Many of which are pathogenic.
Have you ever wondered where this huge quantity of sewage or urban
waste water is disposed off daily? This cannot be discharged into natural
water bodies like rivers and streams directly – you can understand why.
Before disposal, hence, sewage is treated in sewage treatment plants (STPs)
to make it less polluting. Treatment of waste water is done by the
2024-25